Automotive Painting Guide: Bodywork Prep

Believe it or not, there was a time when using lead as a filler when doing automotive bodywork was considered “quick and cheap.” The proper method, then, was called “metalworking,” and it consisted of hammer-and-dollying the metal straight as much as possible; shrinking or stretching other areas of metal as needed, possibly using heat from a torch; and finally filing very small ripples or imperfections from the surface with a Vixen body file. Only a precious few expert “metalmen” can do this today on sheet-metal bodies, whether steel or aluminum. They can also cut out bad sections of the body, hand-form new ones to fit perfectly, weld them in place, and finish the area so you can hardly see, or feel, where the work was done. We don’t expect you to do this, or to even know—let alone afford—someone who can.

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This Mustang fender doesn’t look seriously dented, but the way it’s crumpled around the headlight and bumper flange, plus the fact that it’s an easily removable part, means it would be simpler and smarter to swap this for a good used replacement, rather than try to straighten it.

Someone who was at Hot Rod magazine in the early ’50s told me a funny story at the other extreme. At that time the magazine had a Ford panel truck with its logo painted on its sides, flames on the front, and Von Dutch pinstriping. Someone, somehow, flipped it over and damaged one side. So they took it to a well-known custom shop to have it straightened and repaired. The funny part is that the customizer had some sort of “gun” that could melt and spray lead onto a body. Instead of properly straight-ening the truck, the shop hammered the panels close to shape and sprayed the imperfections full of lead, grinding and filing to contour afterward. The repaired truck looked all right but was noticeably heavier. This was a genuine “lead sled,” a term for a vehicle repaired with copious amounts of lead instead of proper metalwork.

The short cuts that made Hot Rod’s lead sled can still be taken today, but with plastic filler instead of toxic metal. There is nothing wrong with good-quality filler applied properly and sparingly. In fact, there are a few reasons why filler is better than lead. First, it’s much lighter and considerably cheaper and easier to apply—without special tools, talents, or products. Second, applying lead requires heating the sheetmetal with a torch, which warps it. And, third, the leading process involves acids and other chemicals (in the lead, tinning, etc.) that can come back to lift, bubble, or craze the final paint, sometimes years later, if not thoroughly washed and cleaned away. Believe it or not, I have seen more paint jobs (especially on classic or custom cars) marred by bad lead work than by bad filler.

There are many books on bodywork and metal finishing, but here we focus only on very basic body straightening, done primarily with hand tools. Your first necessity is a bodyman’s hammer and dolly. Start with the hammer with a flat, round head on one side and a pick on the other, plus a dolly with one flat and one curved side, such as the second from right. Add more as you go. Do not use other types of hammers on sheetmetal.

Don’t even think about sanding plastic filler level and smooth without using a hard rubber block or the longer board made for this purpose. They now make even longer blocks than these that curve to fit metal surfaces. Always use as long a block as possible to level filler quickly and evenly.

Buy strips of 36, 80, and 180 grit paper made to fit the “longboard” or “filler board.” Cut them to fit shorter blocks. Most bodywork sanding is done with 36 grit. Use 80 grit on high-fill primers and spot putty. The 180 and finer-grit papers are covered under prepping and block sanding in Chapter 6.

Surprisingly, not all dents are readily apparent to the eye. A trained open-palm hand helps locate minor creases and ripples. But one sure way to locate them is to sand the area with a block or board with 80-grit paper. They show up quickly through existing paint, such as these crescent creases common to roof areas.

The pick end of a body hammer is often misused, but it’s good for erasing such creases. Use it to tap along the exposed raised edge, using little more than the weight of the hammer to do the work. Feel the area with the palm of your other hand as you go. When you’re good at this, no filler is needed.

On the other hand, plastic or resin-based fillers were just coming on the market when I first started trying, and thereby learning, to do bodywork. I did experience some cracking, but that was because (1) the fillers weren’t perfected yet, (2) I didn’t properly clean and prepare the surfaces, and (3) without experience and a proper hammer and dolly, I had to apply too much filler over not-straight-enough metal. But it didn’t take that long for fillers to improve, along with my body-work skills. All of my vehicles have minor amounts of plastic filler, some—I’m afraid I must admit—I applied 20 to 40 years ago. These vehicles see plenty of use, most have been repainted more than once, but not one has ever shown any evidence of plastic body filler cracking, lifting, bubbling, checking, or anything else. Nothing. Ever.

Get the Metal Straight

Before you ever lift the lid on a can of filler, you need to get the metal as straight as you can.

Let’s start with the obvious: dents you can see. If any of these come in the category of rips, crumples, or crashes, they’re probably outside the scope of this book. If your car has been whacked so badly the frame is crooked and the doors no longer close properly, you need professional help with big professional tools (like a frame-puller). You are, of course, welcome to tackle any bodywork you want, with whatever tools you have, and you’ll undoubtedly learn something. But that’s not what we’re teaching here.

If the vehicle has sustained some major damage (or rust) that you’re not going to fix with a hammer, dolly, and a little filler, you can take it to a reputable body shop to have the work done. Depending on what they agree to, how much you trust them, and how much you want to spend (or try to save), you might have three options: (1) have them do the big work like frame straightening, panel-patching or replacing, and then “roughing out” any remaining metal-work so that you can fill and block-sand it at home; (2) have them include filler work, which they can do quickly with their professional shop and air tools; or (3) have them also prime and spot-putty the area so that it’s “ready to block and paint.” Unless you know a shop well, they probably won’t do (1) or (2), and if they do (3), you’ve got to trust them because you won’t be able to see what they did or how much filler they used. As we’ve advised, don’t invest in major bodywork unless the car is worth it.

If the wrinkle or crunch damage is isolated to a body part that is unboltable, such as a front fender or door, you can replace it with a decent one from a wrecking yard, or other source. Should it need stripping or a little ding repair, fine. You can do that.

Some metalmen cringe at the thought of using a grinder on sheetmetal, but in a relatively small area like this, it’s the most efficient way to clean the metal. Use a 36-grit disc, do not use much pressure, and don’t make big sparks or turn the metal blue.

In this case, the creases were visible as you sighted down the rear fender, but I sanded them with a 36-grit board so you could see them.

This body had some surface rust under the chrome strips, and the grinder quickly cleans this off. Make sure it’s all gone before you put any filler or paint over it.

The grinder roughs up the surface so the filler bonds better. Be sure to clean more than the dented area, so you won’t have any filler over paint.

Between major damage for someone else to repair and metal you can swap out lies moderate damage to a panel not easily replaced. We’re talking about metal that isn’t cracked or split, folded over on itself, badly stretched, and does not have holes in it (from rust, or otherwise). To start on this type of repair, I’d suggest getting a body hammer with a round, flat head on one side and a dull-pointed “pick” on the other, and a universal dolly with at least one flat side and one curved side. Both last a lifetime, and together they can do a whole lot of metalwork. A couple of other shapes or sizes of dollies would help, but most any flat or curved, easily held piece of heavy metal will do. You should find plenty of other “metal persuaders” in your garage, as well. But, as a general rule, don’t use a ball-peen hammer for bodywork and don’t pound on sheetmetal with anything like a screw-driver blade or center punch. In fact, do not use the “pick” end of the body hammer for anything other than light tapping.

Before talking about straightening metal, let’s consider what doesn’t need straightening. A lot of door-dings, nicks, and deep scratches can actually be filled with today’s good, catalyzed, high-fill primers. There’s some controversy on this point. Certain bodymen dislike adding and block-sanding heavy layers of primer; they prefer to work on bare metal with thin coats of filler. This is a tough call. The advantage of modern primers and catalyzed spot putties is that they can be applied over existing paint. If you don’t need to strip the car to bare metal, this is the most expedient way to fill these minor dings, scrapes and scratches—those less than 1/8-inch deep. If you grind or strip the paint to bare metal to fill such small dings with filler, first (on newer cars) you’ll be removing the factory-applied base coats and, second, creating a rough, jagged edge around the repair that has to be feather-edged, primed, and maybe even spot-puttied to make it level and smooth with the existing paint, anyway. If you’re stripping the car to bare metal, by all means use plastic filler on these very shallow dings, especially if you can see them. Conversely, the rule is to never apply filler over existing paint or primer. PPG claims that filler should be applied over its high-adhesion DP-series sealers, claiming it sticks better than to the bare metal. I’m too old school to try it. See what your paint dealer or professional recommends.

Let’s talk about metal that needs straightening—that means anything that’s 1/8-inch or more out of shape. Start gently. Especially on newer cars, it’s amazing how much you can straighten sheetmetal with your bare hands. Massage it; bump it. On larger dents, see if it pops back out just by pushing on it from the back side. You might be surprised. If it leaves a crescent crease around the edge, try tapping this out with the flat or pick end of your hammer—tap, tap, tap lightly along it, back and forth. Someone caved-in the rear quarter of my son’s ’50 Ford in a parking lot, just behind the door. After removing interior panels, I was able to push it back out from inside with my feet, pressing on it repeatedly until it was all gone. It didn’t even crack the paint. Another time, something fell on the fender of my VW bug, making a crease about an inch deep and four inches long. I massaged this out with the round end of a large plastic screwdriver handle. You can do a lot of bodywork with your hands, and what’s at hand in your garage.

Hammer-and-dolly work is a talent most can acquire with practice. Two things: most dents in sheetmetal cause some stretching of the metal; hammering sheetmetal directly on a dolly stretches it further, as it “squeezes” the metal between the hammer and dolly. When sheetmetal gets stretched too far from its original shape, it begins to “oil can”—that is, to pop in or out as you press on it. You don’t want this. Fixing it can only be done by shrinking the metal, with heat, in some manner that is beyond the scope of this book. (Sheet-metal expands, like most substances, when it is heated, but then shrinks to smaller than its original size when it cools. Don’t ask me why. But that’s why welding or any other severe heating of sheetmetal warps it.)

Hammer-and-dolly work is a talent most can acquire with practice. Two things: most dents in sheetmetal cause some stretching of the metal; hammering sheetmetal directly on a dolly stretches it further, as it “squeezes” the metal between the hammer and dolly. When sheetmetal gets stretched too far from its original shape, it begins to “oil can”—that is, to pop in or out as you press on it. You don’t want this. Fixing it can only be done by shrinking the metal, with heat, in some manner that is beyond the scope of this book. (Sheet-metal expands, like most substances, when it is heated, but then shrinks to smaller than its original size when it cools. Don’t ask me why. But that’s why welding or any other severe heating of sheetmetal warps it.)

Run your open hand, as shown, back and forth over the area to feel the high and low spots in the metal. It takes practice to develop this “feel,” but it is essential to bodywork. I have marked one high area (near hand) and two low spots.

Fortunately I can get a flat dolly behind the high area. To gently tap it down, I hold the dolly firmly next to the crease, and hammer directly on the high spot. This is called “hammer-off-dolly.” Hammering directly on the dolly, especially with force, stretches the metal (by hammering it thinner), which leads to “oil canning,” where the metal pops in and out. Expert metalmen can hammer, shrink, and stretch the surface perfectly straight. We’re not experts (few are). We want to get it as level as possible, with no high spots and only minor low spots.

So when you’re hammer-and-dollying to work out dents, do not start aggressively. Further, it is better to hammer off-dolly rather than on-dolly. That is, to lower a high spot in the metal, place the dolly under a low spot next to it, and push up with the dolly as you tap or hammer down the high spot. With practice you can use this method to lower high spots, raise low spots, or do both at the same time, all the while hammering on the outer surface with the dolly on the inside. Of course there are places where you can’t get your hand or the dolly inside. You might have to remove some body panels, and definitely interior panels, to work them properly. You can gently use the hammer to tap down some high spots without using a dolly behind it, and a dolly can be very effective, by itself, to bump or knock low spots up, or out (even big ones, to start with).

But the main thing is, when you’re straightening sheetmetal with a hammer and dolly (or anything else), try not to stretch it any more than it already is. That’s why you should never beat sheet-metal aggressively with just a hammer (and especially not a peen or a pick) to try to straighten or undent it. You just make it worse, and possibly ruin it. The more you work metal, the harder and more brittle it gets, until it cracks.

Also, since you’re going to be filling the surface after you have straightened it as much as possible, you want all the irregularities to be low spots, with no high spots. That is, you want all the sheetmetal to be at, or slightly below, its original surface level so that you can fill-in just these low spots with the least overall amount of material possible.

One last word here. We’ve been talking about dents in the metal that you can see, even when the body is stripped to bare metal. These are obvious, and you must fix them. A good, experienced bodyman, however, doesn’t look for metal imperfections with his eyes, he feels for them with his hand. To acquire this bodyman’s touch does take some practice. You use the outstretched hand, palm down, and lightly run it back and forth, with a minimum of pressure. Move your hand fairly quickly, just sort of skimming the surface, in a linear motion (straight back-and-forth). As you learn the feel, any high or low spots in what should be flat sheetmetal are immediately detected, even when you can’t readily see them. Further, you can tell where the metal is high and where it is low. If you’re having a hard time developing this feel, try using a clean, dry, unfolded shop rag between your hand and the surface. Experienced bodymen feel the surface before they start hammer-and-dollying it, at intervals during the process to check progress, and then again at intervals as they are sanding it down with filler or primer, until it feels straight and smooth enough to pass the “touch test.” You want to learn how to do this now, during the bodywork and sanding stage, because once you spray the paint, color-sand it, and rub it out nice and glossy, that mirror finish will definitely visually show any dips, bumps, dings, or ripples your eye couldn’t see before, but your hand should have felt.

Fill It

Now do you think you’re ready to open that can of plastic filler? How big a can did you get? You might as well buy a gallon, since it’s more economical, and it lasts for years until you mix it with the hardener to use it. But you better not put that whole gallon on one car. If you do, the majority of it better end up on the floor as shaving particles and sanding dust. In fact, if you’re a first-timer, don’t feel bad if you go through a gallon of this stuff pretty quickly. Put it on, sand it off; put more on, and sand it off a few times before you start to learn how much to apply, and how much to sand off. The typical beginner puts too much on, which is okay, but then sands too much off, which means you have to apply a second coat and sand again. Two or three tries is normal for starters.

Preparing the surface: Only apply filler to clean, bare metal (or fiberglass). “Bare” is self-explanatory, but I must emphasize “clean.” This not only means no paint, dirt, scale, rust, or other obvious crud, but it also means no oils of any kind. You’ve got oil of all kinds in your garage. If you use any air tools (more next chapter) up to this stage, you should oil them, and they spit some of this oil out as they work. And you have oils in your hands. Your sweat has oil in it. So, before you get ready to spread any filler, I suggest you wipe down the work area with lacquer thinner or acetone, and wipe it dry with a clean cloth.

When the area is as straight as you can get it, it’s time to mix filler. I use a piece of heavy-gauge sheetmetal to mix it on, and a screwdriver blade as a stirrer. Especially for beginners, start with small amounts of filler so it doesn’t harden before you’re done spreading it. You can always add more.

Some important things: Squish the hardener in its tube several times to mix it before opening. You have to add the right amount of hardener; adding more than enough won’t make the filler harden faster, it weakens it because the extra won’t ever “dry.” Likewise, be sure to mix the filler and hardener completely (until it’s all one color), because any that doesn’t mix also stays soft.

You want different sizes of spreaders to apply the mixed filler to larger or smaller areas. The big one is rubber, and bends to conform to body contours. The smallest one is for spot putty. If you can’t find a size you need, cut one to the shape you want (as the two black ones in the middle have been).

Here’s another tip: They sell plastic filler-mixing boards, but you either have to clean them with acetone or throw them away. I use a piece of metal because I can wipe it fairly clean with a spreader after each use, and then hit it with my grinder to quickly clean it when too much hardened filler builds up. I’ve been using this same piece—part of a ’56 Chevy fender—for many years.

Second, we have another controversy. Many metalmen shudder at the thought of putting a grinder to sheet-metal. Any grinder with a 24- to 36-grit disc can remove metal in a hurry. This goes against the laws and religion of “metalwork,” as opposed to “bodywork.” I can see their point. Their mantra is to save sheetmetal, not grind it away. On the other hand, they didn’t get the name “body grinder” for no reason. They’ve been a staple in bodyshops and even custom shops for decades. (Of course, they’ve been grossly misused there for decades, as well.) Used carefully and sparingly, a large body grinder with a 7-or 8-inch 24- to 36-grit sanding disc (or the smaller, right-angle, air-driven die grinder type with a 1-1/2- to 2-inch disc) can be a helpful body tool. If you’re not stripping paint by other means, it is the quickest and most effective way to remove paint from an area that needs bodywork. It is also the best way to remove old filler. And it obviously cleans the surface to shiny metal that you’re going to spread filler over. Just don’t overdo it. Clean the metal, don’t gouge or grind it. As soon as you see sparks start to fly, you’re removing metal. If the metal on the body starts to turn blue in spots, you’re not only removing metal, but heating it enough to warp it. Don’t do that.

Here are some tips. I use a slow-speed body grinder (2,400 rpm, as opposed to the normal 5,000 to 6,000 rpm) for two reasons: first, it’s not nearly as aggressive when used as a grinder; but, second, I can also use it for a buffer when I’m rubbing out paint (which I do much more often). Also, I use 36- to 50-grit discs on sheetmetal (actually I use these grits on the small die grinder on sheetmetal, these days, much more often than on the big body grinder). Never use a body grinder, or a rotary grinder/sander of any type, to try to smooth and shape filler. That’s usually disastrous, and we explain why later.

On the other hand, it says right on the body filler can, in the directions, first thing, “Grind/sand surface with 24 to 80 grit sandpaper down to bare metal.” Admitted, it doesn’t say to use a body grinder for this, and I would emphasize “down to bare metal,” not into it, or through it. Many other books, manuals, and bodymen say to “rough up” the metal surface to be filled with 24- to 36-grit paper (or disc) to give a better bonding surface between metal and filler. I would agree with this. Just stop at “roughing up.”

In this case using a medium-size pliable plastic spreader, I apply the mixed filler quickly, but smoothly and evenly, to the dented area. One tip: when “spreading” filler, don’t just wipe it onto the surface, actually press it onto the metal to make sure it adheres.

Just as in mixing the filler, it takes some experience to learn how much to apply at one time. A bit too much is better than too little, because you’re going to sand off as much as possible. But you can always add more; it’s no big deal.

So you’ve got the area that needs bodywork cleaned to bare metal. You’ve hammer-and-dollied it as close to straight as you can, keeping any dips or wiggles below the original surface as much as possible. And you’ve roughed up the surface with a coarse disc or paper. And it’s clean of any oils or grease. You’re ready to mix and spread filler.

First you need something to mix it on. They sell large plastic “palettes” for this, which are fine and easy to hold, but require cleaning with acetone after each use. You don’t want a piece of cardboard or anything that might have wax or other impurities in it. I prefer a piece of sheet-metal. Mine happens to be cut out of a ’56 Chevy fender, and I’ve used it for many years. I scrape it with the squeegee after each use, but it still builds up. So every so often I hold it on the floor with my foot and grind it back down with my grinder. If you reuse a mixing surface, it’s important to keep it free of any small chunks or hard particles that might get in the freshly mixed filler. For a stirrer, I’ve always used a big, old screwdriver blade. Squeegees or spreaders are made in pliable plastic or rubber, in various sizes (get two or three), and they must be wiped clean after each use with a paper towel, possibly with acetone. If the lip of the spreader isn’t clean, it leaves lines or grooves in the filler as you spread it.

Mixing filler properly is extremely important. Unfortunately, there’s no good way to measure amounts. Again, you learn by experience. Basically, it’s a dab of catalyst (hardener) to a glob of filler. Be sure to knead the hardener in the tube to make it uniform before you open it (if liquid comes out before paste, you didn’t squish it enough). The hardener is a darker color than the filler—usually red—and once you have added it, you must quickly, but very thoroughly, mix the two with your stirrer until it’s all one uniform color (i.e., pink). This is very important. Any filler that doesn’t get mixed with catalyst never hardens. You don’t want soft spots, bubbles, or cavities in your body filler.

If you’ve done your hammer-and-dolly work well (i.e., all metal at or below the intended final surface), sand until a few bare spots in the metal just begin to show, as at lower left. (If one or more obviously high spots appear quickly, get out the hammer and dolly again and work them down.) In this case, one low spot (circled) appeared.

After the first board sanding with 36-grit, whether you had to hammer down some high spots or found a low spot or two—or both, apply another thin coat of filler either to those areas or over the whole area, and begin the process again.

The idea is to sand the majority of the filler back off—meaning you have to get the metal pretty straight to begin with. All filler here is less than 1/8-inch thick. Note that no filler covers paint.

You could resand the area with 80-grit and featheredge the paint, but with today’s high-fill primers, it’s more efficient to spray a couple layers of that first, and then start block-sanding with 80-grit.

The chemical reaction of filler and hardener is highly dependent on room temperature. On a warm or hot day, don’t try mixing a big glob. It will probably harden halfway through the job. What doesn’t get evenly spread on the body in time goes in the trash. Mix a smaller amount and start over in a smaller area. In cold climates, on the other hand, the filler can take forever to harden. Bodyshops in such locales usually have banks of heat lamps they can direct on the mixed and spread filler to make it “set up” quicker, so they can start sanding it. Here’s the important part: Adding more or less hardener to the filler does not make it set up faster or slower. Too little hardener, and the filler never fully hardens. This is rare. The more common problem is adding extra catalyst to try to make it harden sooner. The filler can only bond with a set amount of hardener; any extra catalyst remains soft, just like filler that didn’t get fully mixed. Some of you are going to learn this the hard way.

Modern plastic filler is very good if mixed and applied properly. If it’s not mixed properly, for either of the above reasons, hopefully you will find out during the sanding process. If not, it can ruin an otherwise good paint job, either right away, or much later. If you use it wrong, don’t blame the filler.

Finally, one of the most overused and misused terms pertaining to body and paint work today is a “skim coat” of filler. This has somehow come to mean a relatively thin layer of filler over an entire body panel, if not over the entire body of the vehicle. No, no, no! Just as bad as putting too much filler in one place (too thick), is putting too much filler every place—even if it is thin. In general, you want the minimum amount of total material build-up on top of the sheetmetal of your car, including filler, primers, color, clear—everything. That’s a basic rule of painting that gets broken way too often. I know production shops do it; and now lots of custom shops do it to get show-winning body fit and finish as quickly and practically as they can. They look great on the show circuit for a year or so. Or they might get driven a few times a year. But we assume you’re fixing up and painting a car you’re going to use and keep for a while. Don’t build it out of filler. Don’t do what you might see in some magazines or on some TV shows. You’ve got more than a week to finish this project. Spread a thin, even layer of filler over any areas you have body-worked, covering all the sheetmetal in areas that were dented and that you hammer-and-dollied, plus 2 to 3 inches extra around it. That’s all. Avoid filler at the edges of doors, hoods, and other body panels, as well as in doorjambs. It can get knocked or chipped out of such areas. Poor door or body panel gaps or fitment need to be fixed by realignment, shims, or methods that involve welding—not plastic filler. Spread filler only where it’s needed, and then sand most of that off.

Sand It

Sanding is addressed partly here, and more later. When it comes to paint and bodywork, the home hobbyist’s best friend is the “filler board” or “long board.” We’re assuming you are working primarily with hand tools; however, even if you had a shelf full of air sanders of every kind, hand-sanding in long strokes with the filler board is the fastest and surest way to get a flat, level surface on your vehicle’s body. This holds true whether you’re sanding with 36-grit on filler, 80-grit to level high-fill primer and spot putty, or 180-grit over a final coat of primer on the whole body. Never sand filler without some kind of backing block, and the longer the block you can use, the faster the surface gets flat or straight. Body shops use air-powered DA (dual action) sanders on filler almost exclusively. They’re fast and easy to handle for everyday production work, but it takes practice to level filler evenly with one. I finally got one, but I hardly ever use it. Especially for a beginner, it can make your filler uneven in a hurry, especially in large areas. The filler board is only a little slower, but it’s much better at leveling the surface.

But before you start sanding fresh filler at all, you should always knock down the surface with a Surform blade, commonly known as a “cheese grater.” These come in flat or rounded shapes, in various lengths, and you can use them by themselves, or in various types of holders. I recommend the longer ones, one flat and one round. The round one works on flat surfaces (drag it at an angle), and is easier to hold by itself. The flat one bends to concave or convex surfaces, but I also like to use it in a holder with handles, like a wood plane. Use the grater blade after the filler has started to harden, but before it’s fully set-up (when it’s a little harder than rubbery). Don’t be too aggressive with it. Primarily, you want to shave off the “ridges” and uneven surface of the filler and, depending on how much you put on, get it down close to the surface you want, quickly—but not too far. The other major purpose for always using the blade before sanding is that the filler, as it hardens, forms some sort of film on the surface—I don’t know what it is and you can’t see it; it feels a little sticky to the touch. But it clogs your 36-grit sandpaper the minute you start to sand it, and you can’t clean it out. Sandpaper’s too expensive to waste. But if you shave the surface of the filler with a cheese grater blade first, even if it has hardened, it prevents it from clogging the paper.

If you’ve ever seen an article on bodywork, you’ve undoubtedly read that you should first shave the filler with a “cheesegrater” blade. There’s more to it than that. These perforated, sharpened, flat or rounded metal blades were first known by the Stanley trade name “Surform,” and were invented to work, in appropriate holders, like a wood plane. They also work great for shaping or sculpting semi-soft sub-stances such as foam blanks for surfboards. But we don’t want so much filler on your car that it needs sculpting or shaping. The purpose of the cheesegrater in the filler process is twofold—(1) to quickly “knock down” (shave off) any rough edges, overlaps, etc. to smooth the surface, and (2), more importantly, to remove a sticky film that develops on the surface of the filler as it hardens. It’s easiest to “cheesegrate” the filler just before it fully hardens (when it’s like hard rubber). The flat blades curve to a rounded surface (when not used in a holder); either flat or rounded blades can be used with or without a holder/handle. Pull the blade, without much down-pressure, in long sweeps, to shave the high spots off the filler. And even if the filler has hardened, you should “cheesegrate” it; otherwise your sandpaper immediately clogs with sticky, small blobs of filler.

After you’ve shaved the sticky surface off the bondo and leveled it a bit, it’s time to go to work with the “longboard” and some 36-grit paper. Again, let the tool do the work; sand in long sweeps and at angles so the edge doesn’t cut a straight line. Feel the surface with your hand as you go, but you can usually see high spots (they sand first) and low spots (they don’t sand) as you go.

The repaired area looks ready for paint here, but it’s not. The 36-grit leaves deep sanding scratches and the ground paint edges are still not smooth. That comes from block sanding with 80- and 180-grit. But the fender is now straight, with no creases, bumps, or dips.

Not all areas that need bodywork are flat. In the rear corner of the right front fender, where I have “frenched” the aerial, there is a convex crown with a concave area between it and the fender inner edge. Here I am using a rounded cheesegrater, without a holder, in the convex area.

Next I use a strip of 36-grit paper held around a round rubber “block” made for such areas. A straight piece of radiator hose would work as well, and pieces of smaller hose work in tighter-curved areas.

On the crowned part I use a medium-size flat block, but sand at 45-degree angles to the crown. Whenever sanding filler, be careful not to cut grooves or troughs in it.

The last step for professional bodymen is to add daubs of “spot putty” over the primer to fill any bubble holes, deep scratches, or other minor indents in the surface, leaving it to the painter to block smooth (sometimes to the painter’s consternation). Since you’re (probably) the bodyman and painter, do the filler work properly and use spot putty very sparingly.

I have also, surprisingly, seen painters using lacquer putty recently. It’s quick and easy, but it’s nothing more than lacquer primer in paste form. I strongly recommend you use a good, catalyzed spot putty. It only takes a couple of seconds to mix (as shown), and hardens like filler, permanently.

Catalyzed spot putties are usually lighter and easier to sand, but the main difference is that filler is made to go on bare metal, and putty over primer (neither should be used vice-versa).

Besides pinholes and sand-scratches in filler, putty is good for filling typical door-dings and shopping-cart gouges. After scuffing the paint and squirting the area with primer, I am adding putty with a small squeegee to a small, round door ding.

To sand the putty smooth, I wrapped a piece of 80-grit paper around a flat, hard rubber squeegee as a sanding block. Once it’s smooth, re-prime the area before painting.

The photos in this book show various sanding boards, blocks, and tubes. But use the longest practical one, with 36-grit paper, to start working the filler flat. On most surfaces, but particularly on convex-curved ones, I like to sand at 45 degrees, both ways, with the board pointing toward the front of the car (or at 90 degrees, whatever the surface calls for). But you don’t want to sand ruts or grooves into the filler by sanding too long in the same place, in the same direction. Let the long board level the surface.

Keep sanding until you just start to uncover patches or spots of bare metal. Run your hand over the surface, as we described, to feel if these are high spots. If they appeared quickly, in small spots, they probably are. If so, get out your body hammer once again and, depending on their size, tap them down just a bit with the flat or pick end. You probably don’t need a dolly for this. Then mix and spread another “skim coat” of filler over this same area, and sand it with the board and 36-grit once again. Hopefully it will level out (your bare hand should tell you this) before, or just as, you uncover bare metal again.

You don’t have to get it perfect now. But if you get it reasonably level in two tries, you’re doing a good job. You could follow with 80-grit on the board, but most would stop here, and spray an ample coat of high-fill primer over the worked area, before going to 80-grit sanding. If you ground paint away to do the bodywork, feather edge it with 80- or 180-grit paper on a block or with a pad before shooting the primer. We’ll talk more about sanding after this stage in Chapter 6.

Written by Pat Ganahl and Posted with Permission of CarTechBooks

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